218 &OAD ENGINEEBESG
the safest plan is to distribute the weight over the undisturbed
foundation by reinforcing the concrete with a suitable fabric and
bridging the trench by hacking away the sides; alternatively,
cement may be added to the filling.
With a deep trench this might necessitate reinstatement of a
very wide span, and this would obviously be a very expensive opera-
tion. With a shallow trench permanent reinstatement can be per-
formed within a very short time after the filling; if weak or rough
concrete is placed in the trench as filling, the road surface may be
relaid almost immediately.
As an instance of this treatment, a case may be cited where the
only position possible for a new high-tension cable was beneath the
chapnel of a concrete road, the trench being 3 ft wide and about 4 ft,
deep to the top of the cables. Ordinary filling would have left the
subgrade beneath the adjoining slab unsupported, with the in-
evitable result that serious cracking would have ensued. The con-
tractor was required to fill in the trench with a 9 :1 mixture of
clinker concrete to within 4 in. of the surface. Upon this filling rapid-
hardening granolithic concrete, with fabric reinforcement, was laid
by the highway authorities to the cost of the contractor (Fig. 138).
Thus the road was actually stronger than before disturbance.
Some irregularity of the joint with the existing concrete is an
advantage, as this may be arranged to give a greater span in some
parts than in others. Where the original concrete slab has been
reinforced it may be possible to take advantage of the projections of
the steel in order to link up with the new reinforcement.
Another method of reinstating concrete is to lay it some 2 or 3 in.
thicker than the original slab, with the additional thickness passing
under the thinner slab, thus giving a greater bearing area and sup-
port to the undisturbed concrete. The sides of the old concrete may
be stepped or sloped to give a key for the new slab (Pig. 138).
The object of this greater area and thickness is partly to give
greater mass or moment of inertia to resist disturbance or movement
due to traffic loads, and partly to afford support to the adjacent and
original slab. The author has found this method entirely successful
in practice.
The use of square-mesh reinforcement is often justified in repairs
of this kind where additional strength and rigidity is required,
Some years ago, the author laid a concrete road, having a hairpin
bend, carrying heavy traffic under the worst conditions, with this
4-in, square-mesh reinforcement in the surface; it is now taking the
wear which would otherwise be borne by the concrete wearing course,
,and which would have failed long since owing to the twisting and
grinding action of the wheels of heavy vehicles on sharp curves,
Kg. 139,